Integrated optical system for endoscopes and the like

ABSTRACT

Optical systems for endoscopes, borescopes, dental scopes, and the like which are characterized by having three groups of lenses of positive optical power and an external entrance pupil. Typically, all three groups of lenses are displaced from the pupil and focal planes. As a consequence, the displaced groups take part in the image transfer as well as in the pupil transfer. The optical power requirements can thus be shifted from one group to amother, distributing as well as reducing the overall power requirement. Moreover, the aberration correction can also be shared between these groups. The first group, which conventionally has the highest optical power, and consequently a large amount of aberrations to be corrected, can in this way transfer some of the optical aberration correction to the other groups. The sharing of the optical functions and aberration correction results in a fully integrated optical system. The reduction in the total amount of optical power is so large that a line-of-sight deviating prism can be readily accomodated between the entrance pupil and the first lens group. The resulting simplicity of the optical system makes it suitable as a disposable item.

FIELD OF THE INVENTION

[0001] The present invention relates generally to optical lens systems, and more particularly to lens systems suitable for endoscopes and the like.

BACKGROUND OF THE INVENTION

[0002] In endoscopy and related fields, such as borescopes and dental scopes, the complete optical system is thought of as consisting of four basic and separate optical functions. Those functions are, in sequence of the direction of the travelling light as follows: 1. an objective which forms the first image of an object under surveillance, 2. a field lens which images the pupil of the objective onto the next image transfer lens, and 3. an image transfer lens which reimages the first image onto the next field lens. The pupil and image transfer steps are repeated as often as is needed to obtain a desired tubelength. 4. A focussing lens which presents the final image to a sensor, like a person's eye, a CCD camera, or a photographic film. This approach is the classical approach and it is appropriate for the following reasons. 1. The design of the optical system is broken up into parts with single and clearly defined and separate functions, functions to each of which an optical designer may bring considerable experience. 2. The light transfer capacity and information transfer capacity of an endoscope is at a maximum when the optical power is concentrated at the image planes and pupil planes. The expedience of this approach is brought out by numerous U.S. patents on endoscopes which consistently treat the objective, the relay system, and the eyepiece as separate parts of the total system.

[0003] The disadvantage of treating the different optical components as separate entities is that the distribution of the optical powers is very uneven and that certain aberrations are naturally at a maximum, like astigmatism, field curvature, and chromatic aberrations. The correction of these aberrations require relatively short radii. These short radii are difficult to fabricate, require tight tolerances, and they are therefore the main contributors to the considerable cost of the fabrication of an endoscope. A truly inexpensive endoscope, sufficiently inexpensive to be offered as a disposable item, is presently not practical with conventional designs.

SUMMARY OF THE INVENTION

[0004] The present invention provides an integrated optical system suitable for endoscopes, borescopes, dental scopes, and the like which contains a minimum of elements and which elements have relatively long radii and need not be of a meniscus shape. The outside entrance pupil location is very suitable for a tapered probe or for concealment. The entrance pupil distance sufficient to accomodate a line-of-sight deviating prism is a natural consequence of the arrangement of the optical groups. The system leads itself to mass production and is highly insensitive to tilt and decentration of its components. As a consequence it is eminently suitable as a disposable item.

[0005] Broadly, the foregoing advantages are achieved in a lens system which is characterized by an integrated design which has an external entrance pupil and in which the majority of the groups are displaced from the image planes and pupil planes. In this way most components share in the pupil transfer as well as in the image transfer. Moreover, the aberration correction is distributed in an advantageous way over all the groups, providing relief to the first group which conventionally is in need of most of the aberration correction. It has been found that this integration of the optical functions and aberration correction is very beneficial in that it greatly simplifies the optical system.

[0006] A plano-convex lens, or even a double convex lens when used according to the invention is free of astigmatism since it is displaced from the stop location. In this way no optical surfaces of very short radii are needed to correct the astigmatism of the total optical system. Furthermore, the spherical aberration of a convex-plano lens used in the present invention is very near the minimum possible for a single element. Also the chromatic aberration is greatly reduced by the displacement of the elements from the image planes and pupil planes as a comparison with the classical arrangement will readily show. A factor two to four in the reduction of the chromatic aberration is thus achieved without the presence of a chromatic aberration reducing element, sometimes making further color correction unnecessary. Even a system incorporating several transfers is fully color corrected by the use of a single color correcting element. The distortion, which is usually very high in the objective, is corrected at more convenient and effective places. The result is a single integrated system which replaces the three conventional separate parts, i.e. the objective, the field lens, and a relay lens. This single integrated system may be augmented, as is well known in the art of optical design, with additional optics, like a close-up lens, a field expander, a field flattening lens, or with additional relay groups, without falling outside the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an optical schematic view of an endoscope constructed in accordance with a conventional layout in which each component has a single function in the system.

[0008]FIG. 2 is an optical schematic view of a first preferred embodiment in which the entrance pupil is located outside the first group by a relatively small distance.

[0009]FIG. 3 is an optical schematic view of a second preferred embodiment in which full advantage of the power reduction and aberration reduction is taken by locating the entrance pupil outside the first group by a large distance.

[0010]FIG. 4 is an optical schematic view of a third preferred embodiment which incorporates a rodshaped element.

[0011]FIG. 5 is an optical schematic of a fourth preferred embodiment of all glass elements which incorporates a single negative element which provides chromatic aberration correction of the whole system.

[0012]FIG. 6 is an optical schematic of a fifth preferred embodiment of a simple glass & plastic system with full correction of chromatic aberration.

[0013]FIG. 7 is an optical schematic view of a sixth preferred embodiment in which the three basic groups have been augmented by an element near the focal plane of the first group.

[0014]FIG. 8 is an optical schematic view of a seventh preferred embodiment in which a forth element of low optical power has been added near the focal plane of the first group and which contains a single negative element the correct the chromatic aberrations.

[0015]FIG. 9 is an optical schematic of an eighth preferred embodiment which incorporates a meniscus shaped element.

[0016]FIG. 10 is an optical schematic view of a ninth preferred embodiment which incorporates a second image relay and is fully corrected for chromatic aberrations with a single element of negative optical power.

[0017]FIG. 11 is an optical schematic view of a tenth preferred embodiment which incorporates a third image relay and is still fully corrected for chromatic aberrations using only one element of negative optical power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The illustrative embodiments to be described below are standardized to a length of about 100 millimeters of the basic optical system and mostly for a nominal magnification of unity. In this way the performance of the various examples can be conveniently compared. Embodiments with other magnifications, field of views, numerical apertures, and with additional relays are presented in order to show that the general concept of the invention is effective over a wide range of applications. In FIGS. 1 through 11 the focal planes are indicated by an ‘F’ and the pupil planes by an ‘P’. Table I through XI present the constructional parameters of the preferred embodiments. All dimensions are in millimeters. The first column indicates the surface number, the second column indicates the radii, and the third column indicates the axial separations. The refractive indices and dispersion are presented in the usual manner, with respect to the e, F′, and C′ spectral lines. The aspheric data are presented in the standard manner.

[0019]FIG. 1 is an optical schematic of an endocsope which is constructed in accordance with the classical concept of separation of the various functions. Group I is an objective which contains the entrance pupil plane while group II represents a field lens which is located at the focal plane of the objective. Group III represents a transfer lens which transfers the image formed by the objective unto a next focal plane. All groups are located at pupil planes or focal planes. It is apparent from the drawing as well as from the data of Table XII that the distribution of optical power is very uneven. The value of the sum of the absolute values of the curvatures, which is a measure of difficulty of fabrication, is listed in Table XII for this version which is uncorrected for chromatic aberrations. A version corrected for chromatic aberration would have more than double the value for the sum of the curvatures. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table I.

[0020]FIG. 2 is an optical schematic of an endoscope of extreme simplicity. Only three plastic elements on non-meniscus shape and devoid of steep curves are needed to provide diffraction limited performance for the monochromatic aberrations. Many applications do not require a line-of-sight deviating prism and in such cases a pencil-shaped tip, which is often an advantageous configuration, can be readily provided. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table II.

[0021]FIG. 3 is an optical schematic of an endoscope which is also of extreme simplicty in construction and is nevertheless highly corrected for all aberrations, including the chromatic aberrations. Although no negative element has been added to provide correction of the chromatic aberrations, the chromatic aberration is more than a factor four smaller than in the classical layout and is within the diffraction limit. This example clearly shows the gain which a redistrubution of power, with the attendant shift of pupil location brings. This somewhat extreme case is achieved at the cost of a much larger size.

[0022]FIG. 4 is an optical schematic of an endoscope which consists of only two components. The second and third group are cemented to a rodshaped element, thus having only four glass/air surfaces. Despite its simplicity, all aberrations are at the diffraction limit. This example shows that rodshaped elements can be profitably employed in the present invention. It also shows that rodshaped elements may alter the location of the intermediate pupil plane and focal plane, both having crossed over the second and third group. A shorter rodshaped element can put the intermediate planes at the second or third element if so desired. The designs all have in common that no meniscus-shaped are needed. This does not, of course, preclude their use, as is shown in this example. The gain however is modest.

[0023]FIG. 5 is an optical schematic of an endoscope which is entirely constructed of glass elements, none of which are of the meniscus shape. All curvatures are shallow and of a spherical shape only. The first group provides without any strain the needed space for a line-of-sight deviation prism between the entrance pupil and the first group, even in the case of a large field of view of seventy degrees. It is important to note that, despite the fact that the first group is not color corrected in any way, the chromatic aberrations of the whole system are fully corrected by means of a single negative element only. All thre groups are far removed from the intermediate focal plane and pupil plane, showing the full intergration of the three groups. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table V.

[0024]FIG. 6 is an optical schematic of an endoscope which is partly constructed of glass and partly of plastic. Again no steep curves nor meniscus elements are needed to achieve the relatively high N.A. of 0.025. The distortion is well corrected. The object distance has been set at infinite distance to show that the basic design is not affected by a change in magnification as is generally the case with endoscopes. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table VI.

[0025]FIG. 7 is an endoscope which to which one more group of optical power has been added, resulting in a modestly improved monochomatic performance. The added element is positioned close to the image plane of the objective where it is most effective. Its relative weak, in this case positive, power shows that most of the burden of the optical functions as well as the aberration correction is carried by the groups which are displaced from the image planes and pupil planes. This example shows that an additional element near an image plane or a pupil plane is not excluded. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table VII.

[0026]FIG. 8 is a highly corrected endoscope of all plastic elements with a relatively high N.A. of 0.025. Only one of the elements, the second element, favors a position which is close to an image or pupil plane but is again of low optical power. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table VIII.

[0027]FIG. 9 is an endoscope of similar design as the one shown in FIG. 8. The magnification has been increased to 2X, showing that the design remains very similar to the 1X and 0.0X designs, as is generally the case with endoscopes. Again a meniscus element has been employed to show that despite the fact that the present invention can be very well excecuted with non-meniscus elements, their emploment is by no means excluded. In this case the fourth group, the meniscus element is of negative power, again showing that the fourth element is a non-essential addition to the three group concept of the invention. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table IX.

[0028]FIG. 10 is an endoscope to which a second relay has been added. It has a very large field of view of eighty degrees and a relatively high N.A. of 0.025. Despite these large values a deviation prism can be readily accomodate in between the objective and the entrance pupil. The total system is still very well corrected and needs only a single color correcting element of low power in order to provide full correction of the chromatic aberrations. As the first three groups are fully correctible by themselves, the addition of classical relays to those first three groups is not excluded. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table X.

[0029]FIG. 11 shows an endoscope with three image relays and is still very well corrected. Again the chromatic aberrations are fully corrected with a single element of negative optical power. It this case the optical power of the color correcting element approaches a value comparable the those of the other components. All elements are of glass and no aspheric surfaces are employed. The pertinent performance data are listed in Table XII and the constructional parameters are listed in Table XI.

[0030] It is thus evident from these embodiments that the integration of the three groups of which a conventional endoscope exists, the objective, a field lens, and a relay lens, greatly reduces the overall power requirement. The reduction in the overall power requirement naturally reduces the amount of aberrations to be corrected which results in a considerable simplification of the optical system. An additional, and in many cases a very valuable, feature is that the optimal location of the entrance pupil is outside the system. TABLE I SURF RD TH INDEX v-VALUE CC 0 6.51 INPUT FOCAL PLANE 1 −.51 ENTRANCE PUPIL PLANE 2 2.60 1.40 1.4938 57.0 52.0 3 −1.60 3.00 AIR 7.5 4 4.00 3.00 1.4938 57.0 −12.0 5 3.80 33.00 AIR 6 18.00 2.00 1.4938 57.0 7 −24.76 51.64 AIR 8 OUTPUT FOCAL PLANE EFL= −5.518 F/25 60 DEG F.O.V. MAGN = 1.000

[0031] TABLE II SURF RD TH INDEX v-VALUE CC 0 6.00 INPUT FOCAL PLANE 1 .60 ENTRANCE PUPIL PLANE 2 00 1.2 1.4938 57.0 3 1.10 7.4 AIR −.40 4 00 1.5 1.7762 49.3 5 6.30 36.80 AIR 6 00 1.50 1.4938 57.3 7 −11.81 44.85 AIR −3.00 8 OUTPUTFOCAL PLANE EFL= −5.543 F/25 60 DEG F.O.V. MAGN = 1.000

[0032] TABLE III SURF RD TH INDEX v-VALUE CC 0 6.00 INPUT FOCAL PLANE 1 6.00 ENTRANCE PUPIL PLANE 2 00 3.00 1.4938 57.0 3 −4.7 51.30 AIR −.65 4 25.70 7.00 1.4938 57.0 5 −11.70 18.00 AIR −2.90 6 7.00 2.00 1.4938 57.0 7 −13.48 6.72 AIR −56.00 8 OUTPUT FOCAL PLANE EFL= −3.216 F/25 60 DEG F.O.V. MAGN = 1.000

[0033] TABLE IV SURF RD TH INDEX v-VALUE CC 0 6.00 INPUT FOCAL PLANE 1 2.00 ENTRANCE PUPIL PLANE 2 −5.00 1.80 1.4938 57.0 3 −2.10 1.70 AIR −.65 4 5.83 2.00 1.6203 63.1 5 00 48.00 1.8126 25.2 6 00 2.00 1.4938 57.0 1.30 7 7.01 36.50 AIR 8 OUTPUT FOCAL PLANE EFL= −4.846 F/25 60 DEG F.O.V. MAGN = 1.000

[0034] TABLE V SURF RD TH INDEX v-VALUE 0 6.00 INPUT FOCAL PLANE 1 .20 ENTRANCE PUPIL PLANE 2 00 3.00 1.7162 53.2 3 00 1.50 1.7762 49.3 4 −4.0 .20 AIR 5 13.50 1.50 1.7762 49.3 6 −13.50 9.50 AIR 7 00 1.50 1.7762 49.3 8 −10.9 30.80 AIR 9 00 1.20 1.8097 30.2 10  8.80 2.00 1.5914 61.0 11  8.47 42.55 AIR 12  OUTPUT FOCAL PLANE EFL= −5.495 F/33.3 70 DEG F.O.V. MAGN = 1.000

[0035] TABLE VI SURF RD TH INDEX v-VALUE CC 0 1E15 INPUT FOCAL PLANE 1 3.2 ENTRANCE PUPIL PLANE 2 4.90 2.5 1.4938 57.0 −1.50 3 −2.90 18.60 AIR −2.50 4 00 2.00 1.4938 57.0 5 −8.80 24.00 AIR −.70 6 −7.00 1.20 1.5901 29.6 1.40 7 00 2.00 1.4938 57.0 8 6.55 40.51 AIR 9 OUTPUT FOCAL PLANE EFL= −7.794 F/25 60 DEG F.O.V. MAGN = .000

[0036] TABLE VII SURF RD TH INDEX v-VALUE CC 0 6.00 INPUT FOCAL PLANE 1 1.90 ENTRANCE PUPIL PLANE 2 00 2.50 1.4938 57.0 3 2.00 2.70 AIR −.66 4 00 2.00 1.4938 57.0 5 −16.80 25.00 AIR 32.00 6 00 2.00 1.4938 57.0 7 −9.60 31.20 AIR −1.20 8 00 2.00 1.4938 57.0 9 −17.85 24.68 AIR −28.00 10  OUTPUT FOCAL PLANE EFL= −5.301 F/25 70 DEG F.O.V. MAGN = 1.000

[0037] TABLE VIII SURF RD TH INDEX v-VALUE CC 0 6.00 INPUT FOCAL PLANE 1 3.20 ENTRANCE PUPIL PLANE 2 00 2.50 1.4938 57.0 3 −2.50 3.00 AIR −.64 4 00 2.00 1.4938 57.0 5 −26.00 24.70 AIR 57.00 6 00 2.00 1.4938 57.0 7 −9.20 25.00 AIR −1.00 8 −4.30 1.20 1.5901 29.6 −.30 9 00 2.00 1.4938 57.0 10  −3.61 28.35 AIR −.70 11  OUTPUT FOCAL PLANE EFL= −5.599 F/20 60 DEG F.O.V. MAGN = 1.000

[0038] TABLE IX SURF RD TH INDEX v-VALUE CC 0 3.00 INPUT FOCAL PLANE 1 2.40 ENTRANCE PUPIL PLANE 2 12.40 3.00 1.4938 57.0 3 −2.32 9.70 AIR −.80 4 −7.60 2.00 1.4938 57.0 5 −8.10 15.90 AIR 2.80 6 00 2.00 1.4938 57.0 7 −10.00 28.50 AIR 1.20 8 −24.00 1.20 1.5901 29.6 70.00 9 5.00 2.50 1.4938 57.0 10  −6.36 29.82 AIR 11  OUTPUT FOCAL PLANE EFL = −4.891 F/20 60 DEG F.O.V MAGN = 2.000

[0039] TABLE X SURF RD TH INDEX v-VALUE AD AE 0 8.00 INPUT FOCAL PLANE 1 .10 ENTRANCE PUPIL PLANE 2 00 3.80 1.8126 25.2 3 00 1.80 1.7762 49.3 4 −4.30 .20 AIR 5 11.40 1.50 1.7762 49.3 6 −17.00 10.00 AIR 7 00 2.00 1.7762 49.3 8 −18.90 40.30 AIR 9 13.00 2.00 1.8550 23.6 10  8.50 2.50 1.4985 81.2 11  −19.30 39.80 AIR 12  00 2.00 1.7762 49.3 13  8.34 20.00 AIR 14  00 2.00 1.7044 29.8 15  −8.83 14.04 AIR 9.0E-4 2.0E-5 16  OUTPUT FOCAL PLANE EFL= 3.792 F/25 80 DEG F.O.V. MAGN = −.500

[0040] TABLE XI SURF RD TH INDEX v-VALUE 0 12.00 INPUT FOCAL PLANE 1 .10 ENTRANCE PUPIL PLANE 2 00 4.50 1.8126 25.2 3 00 2.00 1.7762 49.3 4 −4.30 .20 AIR 5 38.00 1.50 1.7762 49.3 6 −14.00 15.00 AIR 7 00 1.60 1.7762 49.3 8 −14.00 27.00 AIR 9 50.00 1.20 1.8550 23.6 10 4.50 3.00 1.4985 81.2 11 −4.70 28.80 AIR 12 00 2.00 1.7762 49.3 13 −11.00 26.70 AIR 14 00 2.00 1.7662 49.3 15 −9.90 27.30 AIR 16 −14.70 2.00 1.7662 49.3 17 −8.00 40.70 AIR 18 00 2.00 1.7662 49.3 19 −20.33 50.40 AIR 20 OUTPUT FOCAL PLANE

[0041] TABLE XII 1 2 3 4 5 6 7 8 9 10 11 12 Fig N.A. FOV M EPD El. Relay sC Dist Ptz Wavfr AxClr 1 .020 60 1.0 −.5 3 1 1.62 −2 .077 .79 .90 2 .020 60 1.0 .6 3 1 1.15 −2 .049 .32 .80 3 .020 60 1.0 6 3 1 .55 −1 .022 .10 .21 4 .020 60 1.0 2 3 1 .99 −1 .025 .27 .31 5 .017 70 1.0 1.9 5 1 .84 −16 .031 .31 .12 6 .020 60 .0 3.2 4 1 .96 −3 .046 .46 .14 7 .020 70 1.0 1.9 4 1 .72 −6 .029 .27 .63 8 .025 60 1.0 3.2 5 1 1.06 −1 .023 .21 .31 9 .025 60 2.0 2.4 5 1 1.47 +0 .026 .15 .03 10 .020 80 −.5 2.2 7 2 1.03 −4 .017 .21 .25 11 .017 60 .5 2.6 9 3 1.51 −11 .044 .48 .04

[0042] Column 1 Figure number.

[0043] Column 2 Numerical aperture at the output focal plane.

[0044] Column 3 Total field of view at the input side.

[0045] Column 4 Magnification.

[0046] Column 5 Entrance pupil distance (air equivalent value).

[0047] Column 6 Number of elements with optical power.

[0048] Column 7 Number of image relays.

[0049] Column 8 Sum of the absolute values of all radii.

[0050] Column 9 Maximum image distortion in percent.

[0051] Column 10 Petzvalsum of the total system.

[0052] Column 11 Monochromatic peak to valley wavefront deformation over the whole field and unvignetted aperture.

[0053] Column 12 axial chromatic aberration in waves. 

Having thus described the invention I claim:
 1. An endoscopic optical system extending along an optical axis comprising: at least three spaced-apart groups of optical elements of uniform refractive index disposed serially along the optical axis for the transfer of an image from an input focal plane to an output focal plane, each of said at least three groups being of positive optical power and having at least one element, and means defining an entrance pupil at an entrance pupil plane which is positioned between the input focal plane and the nearest one of said three groups.
 2. An endoscopic optical system of claim 1 wherein a focal plane or pupil plane exists between a majority of the pairs of adjacent ones of said at least three groups.
 3. An endoscopic optical system of claim 1 wherein each of said three groups is spaced from each of the focal and pupil planes.
 4. An endoscopic optical system of claim 3 wherein each of said at least three groups has a maximum radial width and is spaced from at least one adjacent focal or pupil plane by a distance greater than the maximum radial width of that group.
 5. An endoscopic optical system of claim 1 further comprising an optical axis-bending means disposed between said entrance pupil and said one of said groups.
 6. An endoscopic optical system of claim 1 wherein each element in each of said groups has surfaces and each of said surfaces has a curvature, and the sum of the absolute values of the curvatures of all said elements in said at least three groups is less than 1.5.
 7. An endoscopic optical system of claim 8 further comprising a fourth one of said at least three groups having at least one element with surfaces with curvatures, and the sum of the absolute values of the curvatures of said said four groups is less than 1.5.
 8. An endoscopic optical system of claim 1 wherein said at least three groups have elements with positive power which produce an image on an output focal plane having an axial color aberration of less than 0.8 waves.
 9. An endoscopic optical system of claim 1 wherein only one of said elements in said at least three groups has negative power.
 10. An endoscopic optical system of claim 11 wherein only one of said elements in said at least three groups produce an image on an output focal plane having an axial color aberration of less than 0.5 waves.
 11. An endoscopic optical system of claim 1 wherein each of said elements is made of plastic.
 12. An endoscopic optical system of claim 1 wherein each of said at least three groups consists of a single element of positive optical power.
 13. An endoscopic optical system of claim 1 further comprising a rodshaped element.
 14. An endoscopic optical system of claim 1 further comprising a first relay assembly for transmitting an image focussed on the output focal plane to a first downstream focal plane, said relay assembly comprising a plurality of elements, with only one element in said at least three groups and said relay assembly having negative power.
 15. An endoscopic optical system of claim 16 further comprising a second of said relay assemblies for transmitting an image focussed at the first downstream focal plane to a second downstream focal plane, with only one element in said at least three groups and said first and second relay assemblies having negative power.
 16. An endoscopic optical system of claim 1 consisting of said three groups and a fourth group consisting of a single element of a power less than any of the other elements.
 17. An endoscopic optical system extending along an optical axis comprising: three spaced-apart groups of optical elements of uniform refractive index disposed serially along the optical axis for the transfer of an image from an input focal plane to an output focal plane, each group having positive power, having at least one element, and being spaced from each of the focal and pupil planes, each of said elements having surfaces with curvatures with the sum of the absolute values of the curvatures of said elements being less than 1.5, said groups collectively producing an image on the output focal plane, with only one of said elements having negative power and said three groups producing an image at the output focal plane having an axial color aberration of less than 0.8 waves, and means defining an entrance pupil at an entrance pupil plane that is positioned between the input focal plane and the first group, and light travels substantially directly between the input focal plane and said entrance pupil. 